The car brand replaces Saxo Bank, which sponsored the first two years of the awards.
Paul Marshal, general manager of marketing and strategy at Lexus called it, "an excellent opportunity for Lexus to associate itself with the best individuals and companies in the City".
The annual award ceremony, which recognises those who have helped position The City of London at the centre of the world's financial markets, will take place on 17 October.
Lexus is also sponsoring the Personality of the Year award and a new category for Business Innovation.
In addition, investment managers Brewin Dolphin, which is celebrating its 250th anniversary this year, has signed up to sponsor the champagne reception.
Harry Owen, commercial director of City AM, said: "City AM is very proud to have quickly established an event that brings together London's best companies and individuals to celebrate success, innovation and progress in UK business.
"Previous winners illustrate the calibre of the awards and Lexus is a very strong brand that fits naturally with this audience".
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